improve dwarf support on OSX

currently

we dont have stack unwinding support on OSX (libDwarf is ELF format only)

the current way dwarf data is generated on OSX / for macho triggers some segment issue when linking base into a libbase archive, when g1 or g2 is set , but not when g0 is set. that is, even if we had a macho unwinding library for use with GHC, currently we can't even unwind base if we wanted to

currently the stgcrun.c code for darwin/unix envs when build on OSX, can't be built with GCC, because of some mismatch in how llvm vs gcc flavored tools on osx handle dwarf. This means this file, IF we had unwinding support, would need to be built with clang (apple or llvm flavored).

3 a)HOWEVER: on OSX the threaded RTS (when built with clang) is ~15% slower than the same built with GCC.

3 b) so ideally, in a future world with unwinding on OSX, tweaking the build system to make sure that the STGCRUN.c code it always built with clang, or some other fix, that still permits a GCC built RTS OR otherwise does a fix to that GC rts perf issue, would be ideal

Theres a few different action items here, and some of them are quite messy, but they all need to be done for decent perf and debugging experiences on OSX to be in the GHC future!